CO2 signaling in guard cells: Calcium sensitivity response modulation, a Ca2+-independent phase, and CO2 insensitivity of the gca2 mutant

Leaf stomata close in response to high carbon dioxide levels and open at low CO2. CO2 concentrations in leaves are altered by daily dark/light cycles, as well as the continuing rise in atmospheric CO2. Relative to abscisic acid and blue light signaling, little is known about the molecular, cellular, and genetic mechanisms of CO2 signaling in guard cells. Interestingly, we report that repetitive Ca2+ transients were observed during the stomatal opening stimulus, low [CO2]. Furthermore, low/high [CO2] transitions modulated the cytosolic Ca2+ transient pattern in Arabidopsis guard cells (Landsberg erecta). Inhibition of cytosolic Ca2+ transients, achieved by loading guard cells with the calcium chelator 1,2-bis(2-amino-phenoxy)ethane-N,N,N',N'-tetraacetic acid and not adding external Ca2+, attenuated both high CO2-induced stomatal closing and low CO2-induced stomatal opening, and also revealed a Ca2+-independent phase of the CO2 response. Furthermore, the mutant, growth controlled by abscisic acid (gca2) shows impairment in [CO2] modulation of the cytosolic Ca2+ transient rate and strong impairment in high CO2-induced stomatal closing. Our findings provide insights into guard cell CO2 signaling mechanisms, reveal Ca2+-independent events, and demonstrate that calcium elevations can participate in opposed signaling events during stomatal opening and closing. A model is proposed in which CO2 concentrations prime Ca2+ sensors, which could mediate specificity in Ca2+ signaling.